Twin Determination

I’m not talking about stubborn traits which twins have.  I’m sure my twins will be no less stubborn than their older siblings.

I’m only really going to focus on one thing in this post which is to explain how types of twins are determined – in other words, how to know if they are identical vs non-identical.

In my post “Twin Pregnancy Diary – The top 10 twin pregnancy questions I’ve been asked so far”, question 4 was, “Are they identical?” to which I answeredIt seems that they are most likely non-identical”. 

I had no set answer.  

This is why.

Twins can be either monozygotic or dizygotic, identical or non-identical.  

Identical twins are monozygotic.  They are created by a fertilised egg splitting  any time within 14 days of fertilisation.  Depending on when exactly during this time the egg splits will determine whether they will share sacs and placentas.  Identical twins will always be the same sex and have the same dna.  

Monozygotic twins who were created from a fertilised egg splitting from around 9-12 days will share the same sac and placenta and a single chorion and are referred to as Monochorionic/Monoamniotic (“mono” in Greek means “only” or “single”):

(I can tell that you are blown away by my technical drawings).

Identical twins can also be Monochorionic/Diamniotic which means they each have their own sac and placenta and a single chorion.  Again, they will have been created by a single fertilised egg splitting but within 4-9 days of fertilisation:

Our twins are dichorionic/diamniotic which means that they each have their own sac and own placenta.  This is the most favourable type of twin pregnancy to have as your babies are not at risk of getting caught up in each others cords or from TTTS (Twin to Twin Transfusion Syndrome – where one twin takes extra nourishment at the expense of the other and can be very dangerous to both babies).  Here is my technical diagram:

This means that although they are more likely to have been created by the fertilisation of two separate eggs resulting in non-identical twins (or fraternal twins), they might also have been created by a single egg splitting within the first three days of fertilisation resulting in identical twins, hence the reason for my answer above.
How would be know for certain?  
Well, unless a sonogram during pregnancy revealed the genders as being boy/girl twins, which would immediately tell us that they are non-identical as identical twins cannot be different sexes, we couldn’t know for certain until they were born and they were either visibly dissimilar, or on testing their DNA or by examining the placentas.
I hope I’ve managed to explain that clearly and please, tempting as it may be, please don’t steal my drawings for putting in medical journals (although a link back is always nice to have!).



Twin Pregnancy Diary is available to buy in paperback now.

You can read the rest of our Twin Pregnancy Diary available on Kindle at Amazon.


2 thoughts on “Twin Determination

  1. Oh the crazy questions I got with the twins and still do. I still love the one I still get with my 6 year old boys. They ask are they twins, Yes Identical lol Then they will turn right around and ask me if they are related??? What what really????? Congrats on the twins, I cannot ever imagine not experiencing mine, although hard and twice the fun. All I can say is schedule schedule schedule!!! Michelle frugalredneck

  2. You explained this so well! I run into twins all the time that are obviously identical and their parents tell me that they are fraternal because they were in separate sacs! And when I mention that identicals can actually be in separate sacs as well, they say that their doctor told them that separate sacs means fraternal! Why are doctors so mis-informed?! I'm actually an identical twin and have identical twin girls – doubly blessed! Which leads to the question I get all the time… "I thought twins skipped a generation?" My response: "Identicals aren't hereditary! I'm just one lucky momma!"

    Just read your great post on Multiples & More! Congrats on your twins!! Hope you have a long and boring pregnancy (meant in the nicest way!)

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.